The invention relates to a method for operating a photovoltaic system with a plurality of photovoltaic modules and with a first DC motor connectable to the photovoltaic modules, with the motor axle configured for coupling to a shaft of a three-phase generator, wherein the three-phase generator can be connected to a power grid. This method is intended to feed the AC current produced in a large system by the three-phase generator into a power grid, preferably a public utility grid. The system has, in particular, a power rating of more than 1 MW, i.e., a relatively high power output.
The invention also relates to a photovoltaic system for carrying out the method.
In conventional photovoltaic systems, the total DC current supplied by the photovoltaic modules is converted by an electric inverter or converter into an AC current or a three-phase current, which is then supplied to a power grid. Currently, electric inverters based on semiconductor components are commercially offered for large-scale systems, which are designed for a power output of up to 700 kW. Several purely electric inverters must be provided in a photovoltaic system with a power output in excess of 1 MW. For example, a solar system having a power output of 2.5 MW employs at least 8 electric inverters, each having a power rating of 330 kW.
DE 100 61 724-A1 discloses providing a plurality of electronic inverters which are controlled by a relatively complex control process using a plurality of switches at an operating point which is optimized for the individual inverters.
Although an electric inverter has a high-efficiency for converting energy, it should be noted that the inverter reacts very sluggishly due to the relatively large electric capacitance of the employed capacitors. Currently, it may take 20 seconds to 3 minutes, until the associated control device adapts such inverter to changes in the actual incident solar energy.
The invention is based on the concept that a combination of a DC motor with a three-phase motor arranged between the photovoltaic modules and the power grid provides certain advantages in larger systems of 1 MW and above. DC motors and three-phase generators for such high power levels are commercially available. Such a combination has the advantage over a plurality of electric inverters that the smaller number of components makes them less susceptible to statistically occurring malfunctions. Another advantage is reduced maintenance. Advantageously, only two units need to be monitored during operation, namely the DC motor and the three-phase generator.
DE 20 2006 002 726 U1 discloses a motor-generator assembly for operating a solar system. This document, however, only addresses the mechanical construction of the motor-generator assembly. Several pole wheels, which each run on their own stator, are provided on a common shaft. This measure is intended to increase the efficiency of converting the regenerative energy, which is also intended in DE 100 61 724-A1. However, it is not an object of the present invention to increase the efficiency of the energy conversion.
It has proven to be difficult in practical applications to start up an electromechanical machine assembly, which includes a DC motor and a three-phase generator, because particularly in the morning, the required minimum energy necessary for bringing the machine assembly to a rotation speed necessary for synchronization is inadequate, or because synchronization takes a long time. An electromechanical machine assembly of, for example, 4 MW has a weight of approximately 10,000 kg and hence a correspondingly high mass to be brought up to the rotation speed. These masses must be brought to the rotation speed by the solar system, in particular during startup in the morning, to enable synchronization with the grid. In addition, a higher consumption of commutator or contact brushes in the DC motor can be expected with a slowly rotating machine assembly. This causes in the end undesirable wear.
Based on this observation, it is the object of the present invention to provide a method for a photovoltaic system having a high power output which shortens the startup phase and attains the synchronization conditions more quickly, and also reduces wear of the commutator brushes. It is also an object to provide a photovoltaic system which satisfies these conditions.